Transmission electron microscopes (TEMs) have been used in a variety of scientific disciplines for 50 years. A TEM works in a way fundamentally similar to a light microscope except for the use of an electron beam instead of light. Electrons have a much shorter wavelength and consequently allow viewers to see features approaching atomic size (<1 nanometer) in comparison to a limit of 100 nm (0.1 μm for light).
Over the last 50 years, transmission electron microscopy (TEM) technology and its applications have undergone tremendous advances. TEM has become an important analytical tool in many disciplines that require visualization and/or imaging of features less than one nanometer in size, a resolution at least two orders of magnitude greater than that available to optical microscopy.
Unfortunately, the preparation of samples for TEM analysis is fraught with difficulties. TEM only works with samples made thin enough to be transparent to electrons, a distance on the order of 50 nm. Thinning samples through cutting or grinding is especially difficult for hard, tough, and especially brittle materials; furthermore, attempts to meet even reasonable standards of reproducibility and throughput often fail. The bottom line is that successful TEM sample preparation generally requires the use of highly trained, reliable, experienced, and hence very expensive technical personnel.
There are many practical difficulties inherent to using a transmission electron microscope (TEM) to image microstructures. Samples must first be thinned to an ultra-thin membrane transparent to electrons: approximately 50 nm (0.00005 mm) or less. This is less than 1/1000 of the diameter of a human hair. In reality, this is extremely difficult to accomplish on hard materials since many crystalline materials, such as silicon, do not lend themselves to conventional cutting or grinding techniques. Additionally, one cannot easily observe the area of interest while thinning it, and handling the minuscule samples requires a highly trained, dexterous technician.
In the past ten years, the market for TEMs has been static at about 250 units per year (100 in the U.S.). As Government funding decreased for life sciences in general, demand for biological TEMs declined. Offsetting this trend, high-end analytical TEMs for materials (metals, ceramics, semiconductors, superconductors, etc.) increased, largely due to the need to view crystalline structures and thin film interfaces at the highest possible visual acuity. The demand for TEM images continues to grow precipitously in the semiconductor market segment as companies move to smaller design rules and deeper vertical integration.
The increasing demand for TEM does not necessarily translate directly into instrument sales, since the real bottleneck in productivity continues to be the tedious and time-consuming nature of sample preparation. Several labs have commented that their TEMs sit idle much of the time, waiting for quality samples of the right area to be prepared.
There are significant differences in requirements between customers who extract samples from full wafers vs. customers who analyze samples from bulk materials. The bulk sample customers represent a wide cross-section of analytical electron microscope users in a broad range of materials applications in institutional and industrial settings. Although in the majority, they do not have the same critical need for productivity that semiconductor customers have. The wafer customers, best represented by process development and yield engineers, have the following general requirements: 1) they want cross sections of specific devices or defects extracted directly from wafers; 2) they want specific location coordinates transferred from other equipment; 3) they want sample preparation, from full wafer to imaging, to be as fast as possible; 4) they want confidence that an exact location on a wafer can be sampled; 5) they want a minimum of artifact formation; 6) they want the process automated to the fullest possible extent; and 7) they do not want to sacrifice the entire wafer for a single sample preparation.
Within the current general approach of TEM sample preparation, there are subtle variations; however, each consists of a technique for removing a bulk sample from the wafer, reducing the area of interest to an ultra-thin membrane, and finally transferring the membrane to the TEM. The current approaches to TEM sample preparation include using high precision diamond saws, broad beam thinning techniques and single beam focused ion beam (FIB) milling methods. In most these approaches, the sample must be manually transferred to the TEM for analysis.
High precision diamond saws are currently available for TEM sample preparation. Each claims to be able to reach the right spot on a wafer automatically and create an approximate 1 μm×1 μm block containing the sample. The process takes anywhere from 30 minutes to two hours, depending upon the skill of the technician. Clearly, the major drawback to this type of apparatus is the fact that the wafer is sacrificed, and only one sample per wafer is utilized.
The broad beam thinning technique of TEM sample preparation is virtually obsolete for full wafer sample extraction simply because it is slow, the area of interest cannot be targeted, and the ultimate thickness is variable.
There are two different single beam FIB approaches for TEM sample preparation. In one approach, multiple samples can be automatically processed up to, but not including the final cut. After thinning, samples must then be manually transferred to a support grid and into a TEM holder. Some commercial TEM sample preparation methods offer a detachable tip “FIB-EM” that can be pre-mounted on the FIB stage before thinning in an attempt to reduce transfer damage.
A second approach for single beam FIB sample preparation allows users to insert a TEM sample rod into an FIB through an airlock, thus avoiding manual handling of the delicate thin section any time after it is milled. This reduces the chance of physical damage.
The use of a dual beam (combination FIB and SEM) for sample thinning and extraction has become the dominant technique for wafer applications. The area of interest of the wafer can be located using navigation software, and ultra-thin sections can then be cut directly from the wafer. This technique completely avoids the intermediary diamond saw and lapping steps. Unfortunately, the ultra-thin membrane itself must be lifted from the wafer and transferred to a grid manually in this approach.
There are three methods available for transferring the membrane from wafer to TEM sample holder. All three permit the user to prepare multiple samples from one wafer without destroying the rest of the devices on the wafer. One approach allows for picking up thin sections of TEM samples by electrostatic attraction. In this method, FIB prepared samples are manually located under a binocular microscope. The membrane is then touched with a charged micro-manipulator probe in the hope that static attraction between the thin membrane and the probe will occur. Even with an expert technician at the controls, membranes are likely to disappear or crumble during the manual transfer.
A second technique involves grabbing the membrane inside the dual beam with a micro-tweezer. If the membrane survives the detachment from the substrate and tweezer jaws, the operator must then gingerly place the membrane on a TEM grid. The TEM grid must then be transferred to the TEM sample holder. Although there is better visual observation possible at the extraction site, there is also significant increased physical handling.
The newest technique was developed by Tom Moore, and is called the Moore Technique. In that approach, a probe is actually welded to the finished membrane inside the FIB using the FIB's metal deposition capability. The FIB then cuts the membrane free from the matrix. It can then be transferred to a grid, where the probe weld is cut, and the membrane can then be welded to the grid.
Needs exist for a TEM sample preparation that is simple, cost effective, and automated to decrease the risk of human error when transferring samples to a TEM for analysis.